A basic form of Class D amplifier is illustrated in FIG. 1A. Two transistors Q.sub.A and Q.sub.B are driven by a transformer to switch on and off 180 degrees out of phase with each other. The two transistors in combination are equivalent to the double-pole switch illustrated in FIG. 1B, and at their common node produce a square-wave output similar to the waveform illustrated in FIG. 1C.
Ideally, a Class D amplifier should be 100% efficient, i.e., no power should be consumed in transistors Q.sub.A or Q.sub.B as they are repeatedly switched on and off. In reality, however, transistors Q.sub.A and Q.sub.B have their own on-stage resistances, commonly known as the on-resistance of the transistor. They also have inherent capacitances, illustrated as capacitors C.sub.A and C.sub.B in FIG. 1A, which permit charge to build up when either transistor is turned off. Therefore, a voltage difference exists across the transistor when it is turned on, and the resulting current flow through the transistor dissipates energy as heat. This energy loss can be expressed as cv.sup.2 f, where c is the inherent capacitance of the transistor (the value of C.sub.A or C.sub.B), v is the DC input voltage (V.sub.DD) and f is the frequency at which the amplifier is driven. In reality, high frequency Class D amplifiers typically operate at an efficiency of about 50% to 60%. A large portion of this efficiency loss is attributable to the inherent capacitances of the transistors. This inefficiency has seriously limited the suitability of Class D amplifiers for electrodeless discharge lamps and other devices in which significant power losses cannot be tolerated.
In actual operation, transistors Q.sub.A and Q.sub.B having the same electrical characteristic, do not switch off and on instantaneously and simultaneously as implied in FIG. 1C. Rather, the input from the transformer is typically sinusoidal and turns each transistor on when it reaches its threshold voltage V.sub.th, as indicated in FIG. 1D. Accordingly, there is a lag between the time when one of the transistors turns off and the other transistor turns on. The output signal is therefore not a perfect square wave but instead has sloped transitions as illustrated in FIG. 1E, the time lag being denoted as .DELTA.t. Moreover, a finite time is required to turn each transistor on or off.